Towards Quantitative Interpretation of Fourier-Transform Photocurrent Spectroscopy on Thin-Film Solar Cells

Jakub Holovský,Martin Müller,Franz-Josef Haug,Brianna Conrad,Michael Stuckelberger,Amalraj Peter Amalathas,Tomáš Finsterle

doi:10.3390/coatings10090820

Abstract

The method of detecting deep defects in photovoltaic materials by Fourier-Transform Photocurrent Spectroscopy has gone through continuous development during the last two decades. Still, giving quantitative predictions of photovoltaic device performance is a challenging task. As new materials appear, a prediction of potentially achievable open-circuit voltage with respect to bandgap is highly desirable. From thermodynamics, a prediction can be made based on the radiative limit, neglecting non-radiative recombination and carrier transport effects. Beyond this, more accurate analysis has to be done. First, the absolute defect density has to be calculated, taking into account optical effects, such as absorption enhancement, due to scattering. Secondly, the electrical effect of thickness variation has to be addressed. We analyzed a series of state-of-the-art hydrogenated amorphous silicon solar cells of different thicknesses at different states of light soaking degradation. Based on a combination of empirical results with optical, electrical and thermodynamic simulations, we provide a predictive model of the open-circuit voltage of a device with a given defect density and absorber thickness. We observed that, rather than the defect density or thickness alone, it is their product or the total number of defects, that matters. Alternatively, including defect absorption into the thermodynamic radiative limit gives close upper bounds to the open-circuit voltage with the advantage of a much easier evaluation.

Highlights

Sub-bandgap absorptance spectroscopy is a relatively simple defect quantification method, which is well established in the material science of thin-film photovoltaic (PV) materials, such as microcrystalline silicon [1], hydrogenated amorphous silicon (a-Si:H) [2], organic semiconductors [3] and recently hybrid perovskite materials [4,5,6,7]
Using an optical ray tracing model [26], we demonstrate in Figure 2a (ii) that the shape of the absorptance, that is attributed to FTPS signal, is very close to the shape of the absorption coefficient in the low absorption region, especially between 1.2 eV and the beginning of absorption edge (Urbach slope)—in our case at 1.55 eV
We have demonstrated that FTPS spectra used in conjunction with the absorption coefficient defects that are present

Summary

Introduction

Sub-bandgap absorptance spectroscopy is a relatively simple defect quantification method, which is well established in the material science of thin-film photovoltaic (PV) materials, such as microcrystalline silicon [1], hydrogenated amorphous silicon (a-Si:H) [2], organic semiconductors [3] and recently hybrid perovskite materials [4,5,6,7]. Whereas other methods, such as conductivity or photoluminescence, are difficult to interpret and may give results affected by transient effects, sub-bandgap absorptance. Deflection [2] and Photothermal the Constant the era of a-Si:H, two low-absorptance techniques wereSpectroscopy established as(PDS)

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